NASA Detects X-ray Burst From Cosmic Explosion Which Released As Much Energy in 20 Seconds As the Sun Does in 10 Days
Astronomers have observed a record-breaking flash of X-rays caused by a thermonuclear explosion in space, which released as much energy in 20 seconds as the sun does in around 10 days.
The blast occurred on the surface of a pulsar—a rapidly rotating neutron star with powerful magnetic fields that shoots out beams of radiation from its poles, often likened to an interstellar lighthouse. Neutron stars are the incredibly dense remnants of stars which have exploded as supernovae after running out of fuel toward the end of their lives.
The pulsar in question—known as SAX J1808.4-3658 (or J1808 for short)—is located around 11,000 light-years away in the constellation Sagittarius, spinning away at a staggering 401 rotations per second, according to NASA.
On August 20, astronomers detected a sudden spike of X-rays originating from J1808 at around 10 p.m. using NASA's Neutron star Interior Composition Explorer (NICER) telescope on the International Space Station.
NASA says that this is the brightest X-ray burst ever detected by NICER since the telescope began operating in 2013. Furthermore, it produced several phenomena which have never been seen together in a single flash, according to a study published in The Astrophysical Journal Letters.
"This burst was outstanding," Peter Bult, lead author of the study from NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland, College Park, said in a statement.
Among the most unusual elements of the explosion, the astronomers observed that there was a temporary pause in the increase of X-ray brightness. Following this pause, the X-rays started to become brighter again. The scientists are not clear exactly why this occurred but have proposed a potential explanation.
"We see a two-step change in brightness, which we think is caused by the ejection of separate layers from the pulsar surface, and other features that will help us decode the physics of these powerful events," Bult said.
The type of thermonuclear explosion that the scientists observed is known as a Type I X-ray burst. These phenomena occur in the surface layers of neutron stars which accumulate hydrogen and/or helium from a companion star.
In the case of J1808, the pulsar is part of a binary (twin) star system where the other body is a brown dwarf—a class of astronomical objects larger than giant planets but too small to be stars.
The extreme gravitational pull of the pulsar sucks in hydrogen gas from the brown dwarf, which accumulates in a disk around the dense star remnant.
Every few years, this gas disk around the pulsar becomes so dense that atoms in the gas lose their electrons, or in other words, they become "ionized."
This causes radiation to become trapped, starting a runaway process of heating and more ionization. Eventually, the hydrogen gas starts spiralling downwards into the pulsar, forming a hot "global sea."
When temperatures and pressures at the bottom of this ever-deepening sea increase to a high enough point, hydrogen nuclei begin to fuse and form helium nuclei, producing vast amounts of energy. This process is the same as the one that takes place in the core our Sun, fueling the star.
"The helium settles out and builds up a layer of its own," co-author of the paper Zaven Arzoumanian from Goddard said in a statement. "Once the helium layer is a few meters deep, the conditions allow helium nuclei to fuse into carbon. Then the helium erupts explosively and unleashes a thermonuclear fireball across the entire pulsar surface."
